1. Technical Field
The disclosure relates to a semiconductor device and a fabrication method thereof.
2. Description of Related Art
The metal oxide semiconductor material in thin film transistors (TFTs) of conventional liquid crystal displays (LCDs) mainly uses hydrogenated amorphous silicon (a-Si:H). Since some shortcomings of the a-Si:H, such as low carrier mobility (0.1-1.0 cm2/Vs), light sensitivity, and low chemical stability, the a-Si:H has been replaced by low temperature polysilicon (LTPS). LTPS can improve the problem of the low carrier mobility to increase the carrier mobility to 100-200 cm2/Vs, and also has good electrical stability. However, the LTPS still has shortcomings of drifted threshold voltage, complicated process, and expensive cost.
Recently, since transparent oxide semiconductor (TOS) has advantages of high transparency, wide band gaps, high carrier mobility, and easy preparation, TOS has become a widely studied object of scientists. When the TOS is applied on the metal oxide semiconductor layer of TFTs, in addition to increasing aperture ratio, improving utilization efficiency of backlights, and reducing power consumption, a single pixel area also can be reduced to increase the LCDs' resolution.
N type indium gallium zinc oxide (IGZO) is taken for example below. Comparing a-IGZO TFT and a-Si TFT, the carrier mobility of the a-IGZO TFT is 20-50 times of that of the a-Si TFT, and the conducting current is 20 times of that of the a-Si TFT. Therefore, the power consumption of the a-IGZO TFT is much less than that of the a-Si TFT.
However, the current studies show that the touch of water, oxygen and light on the a-IGZO thin film can affect the properties of TFT. Therefore, how to protect the a-IGZO thin film becomes an important issue. At present, the passivation layer of TFTs mostly uses a thin film of silicon oxide or silicon nitride to prevent the penetration of water and oxygen, and the irradiation of light. However, the thin film of silicon oxide or silicon nitride cannot effectively block the light in the visible to the UV region to irradiating on the a-IGZO thin film. Therefore, this becomes an urgent problem.